\section{Introduction to spacecraft attitude stabilization}

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Typically, the purpose of putting a satellite in orbit is to point an instrument (called the payload) at something.
\begin{itemize}
    \item For example, for a communications satellite, the transmitting and receiving antennas need to point toward the Earth.
\end{itemize}
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\begin{center}\includegraphics[scale=0.8]{fig_6_1.jpg}\end{center}
\begin{center}\textcolor{blue}{Figure \arabic{section}.1:} Communications satellite\end{center}
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\begin{center}\includegraphics{fig_6_2.pdf}\end{center}
\begin{center}\textcolor{blue}{Figure \arabic{section}.2:} Typical spacecraft control system\end{center}
The control system typically consists of three parts: \\
\begin{description}
    \item[\textcolor{blue}{Navigation}-] where am I?
    \begin{itemize}
    \item determines the current spacecraft attitude from attitude sensor measurements
    \end{itemize}
    \item[\textcolor{blue}{Guidance}-]where do I want to be?
   \begin{itemize}
    \item determines what the desired spacecraft attitude is, and what the error in attitude is (by using information from the Navigation part).
   \end{itemize}
   \item[\textcolor{blue}{Control}-]how do I get there?
   \begin{itemize}
    \item determines what torque should be applied to the spacecraft to correct the attitude error, and sends this as a command to the attitude actuators.
   \end{itemize}
\end{description}
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What it means is that the attitude (orientation) of the spacecraft needs to be stabilized to some desired attitude.
\begin{itemize}
    \item This can be done both actively and passively, depending on the desired attitude, and how accurately it must be maintained. \\
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    \begin{description}\setlength{\itemsep}{5pt}
        \item[Passive control]: Makes use of the spacecraft’s natural dynamics to ensure a stable equilibrium at the desired attitude, and are useful when accuracy requirements are coarse.
        \item[Active control]: Actuators capable of affecting the attitude are installed on the spacecraft.
        \item[Passive + Active control]: In many cases, passive control method are augmented by active control method.
    \end{description}     
\end{itemize}
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